The present invention relates to a method of controlling the operation of a burner for heating the liquid glass feeders coming from a glass furnace.
In a continuous glass manufacturing line, the glass is melted in relatively large capacity furnaces that deliver molten glass as output. In certain industrial furnaces, such as glass furnaces for hollowware, the molten glass must be conveyed right to the glass-forming machines. To transport this molten glass, “feeders” or “forehearths” lined with refractory materials are used. As the glass is being conveyed in this way, it is cooled and also conditioned so that, on leaving the feeders, its temperature is perfectly stable and homogenous to within ±1° C. To achieve this, the temperature of the glass leaving the feeders must therefore be constant but also perfectly uniform transversely, that is to say over the width of each feeder.
It is essential to control the heat transfer method at the surface of the glass over the entire length of the feeder in order to reduce the output temperature gradient. To do this, it is common practice to equip the feeders with a heating device, which heats by combustion of an air/combustible gas mixture above the free surface of the stream of molten glass. This combustion is obtained using air/fuel burners. While the molten glass is flowing, in order for the temperature of the molten glass to be both lowered and homogenized, series of burners are distributed over the entire path of the molten glass. Owing to the number of burners and the difficulty of detecting and controlling the volumes of flue gases that they create, the combustion may be carried out by burners whose oxidizer is cold air; now, these burners have a generally mediocre efficiency and offer little flexibility as regards obtaining a good transverse thermal profile.
To solve these problems, the combustion of an air/combustible gas mixture has been replaced with combustion of an oxygen/combustible gas mixture using oxyfuel burners. This modification has increased the glass production capacity, and also the combustion efficiency and radiative transfer. Such burners have been described, for example, in Documents U.S. Pat. No. 6,431,467 and U.S. Pat. No. 5,500,030. These burners have in particular the advantage of providing a large operating range, that is to say the possibility of varying the power—and therefore the fuel and oxidizer flow rates—much more widely than in the case of air/fuel burners. Furthermore, the length of the flame of these burners is constant over their entire operating range. This property allows them to heat the edge of the feeders, at the point where the glass cools upon contact with the refractories. They also limit the thermal gradient, and therefore the difference in viscosity, between the core of the feeder and the edges; thus preferential flow of the glass at the centre of the feeder is limited. Moreover, the heating power for a section of feeder by oxyfuel combustion or with oxygen-enriched air is greater than that which can be achieved in air/fuel combustion. The wide power range within which the oxyfuel burners operate allows dynamic regulation which rapidly compensates for the variations in the process and stabilize the glass temperature. The feeders may be equipped over their entire length with several heating zones; in this case, the oxyfuel burners provide great operating flexibility thanks to greater precision in the temperature regulation. If the entire length of the feeder is fitted with oxyfuel burners, this operating flexibility is even greater. Furthermore, the gas consumption is reduced. Oxyfuel burners also allow the volume of flue gases to be reduced, which may in certain cases lead to a reduction in the fly-off and volatilization of certain components conveyed in the feeders, such as pigments.
However, this oxyfuel combustion may have certain drawbacks. Firstly, the flame geometry of the feeder burners is particularly important as it is necessary to ensure that the glass stream heating profile is particularly stable and uniform. However, the thermal behaviour of the materials that make up the self-cooled oxyfuel burners is generally difficult since the ambient temperature therein is generally high, whereas the gas and oxygen flow rates in each burner are low (low unit power). Thus, to ensure a stable flame profile, there is not as much operating flexibility for these burners as the oxyfuel would allow. In addition, the low-speed flow of the burners may be the source of burner failures requiring maintenance. This is because the burners are cooled by convection with the flow of both the oxidizer and the fuel that they use. In the case of combustion with oxygen, the flow volume is about 70% less than that of combustion with air. The cooling is therefore less effective for the same power. The combustion flame with oxygen is also hotter and more radiating. In addition, at low power, the heating of the burner end-fitting may cause premature cracking and therefore as a consequence rapid fouling and premature wear of the burner. Finally, the feeders must always be at an overpressure, and this pressure is maintained by the volume of the burner flue gases. In aerocombustion, this volume is stabilized—a set of flue gas discharge dampers allows the pressure to be adjusted, which it is necessary to monitor and regulate. In oxycombustion, the volume of flue gases is much lower, and in addition, varies greatly with the power, thereby making it difficult to control the pressure in the feeders. A pressure-stable method independent of the instantaneous power conditions is therefore sought.